Method of chromium electrodeposition



United States Patent 3,247,083 METHOD OF CHROMPUM ELECTRODEPOSITION Louis W. Raymond, 50 Broekbend Road, and Roger E. Reath, 1484 Broolrside Drive, both of Fairfield, Conn. No Drawing. Filed Apr; 1, 1964, Ser. No. 356,643 1 Claim. (Cl. 204-51) This invention relates to chromium plating and, more particularly, to a method for electrodeposition of chromium from a chromic acid plating bath using an anode having a titanium body structure at least partially covered with lead to provide an effective anodic area thereof; and

this application is a continuation-in-part of our copending application Serial No. 80,015, filed January 3, 1961, now abandoned.

It is general practice in the chromium plating art to use anodes of all-lead construction in chromic acid electrolytic baths because of their high resistance to corrosion and chemical attack by the acid solution. In view of this and other advantages, all-lead anodes have been widely employed in spite of the fact that they possess mechanical and structural deficiencies which heretofore were considered unavoidable. For example, conventional lead anodes must be relatively massive in order to conduct the high currents required for chromium plating, and it is therefore difiicult to locate them adjacent to some of the more inaccessible surfaces of irregularly shaped articles to be plated. Also, sections of a lead anode which are too thin tend to become warped and deformed during use, with obvious adverse effects to the plating operation, because of their low strength and high weight and also because of over-heating from current flow particularly where the anode is covered with insulation.

The primary purpose of this invention is to provide a new method for electrodeposition of chromium from chromic acid electroplating baths with an anode which has the advantages of conventional all-lead anodes but which does not possess any of these mechanical shortcomings. The anode used in the method of this invention can provide greater strength and rigidity with less required weight and size, and resist corrosion and other chemical attack to an extent at least as satisfactory as in typical lead anodes. Furthermore, this anode has a versatility unknown heretofore in that it can be made in very thin and irregular shapes to plate the more hardto-reach surfaces of a wide variety of articles. Moreover, it is superior to conventional lead anodes in preventing contamination of the electrolyte by the anode material or by matter originating from the electrolytic solution.

Broadly stated, the method of this invention for producing chromium electrodeposits on a base metal comprises the steps of immersing as a cathode at least a portion of the base metal in a chromic acid electrolytic solution and positioning a nonconsumable anode in the solution adjacent to the cathode. The anode used therein has a titanium body structure at least partly covered with a layer of lead in electrical contact with the body structure whereby the lead covered area serves as an effective anodic area. After completing the electrodepositing circuit, chromium is electrodeposited onto the base metal.

Titanium when immersed in an electrolytic bath forms a protective oxidic film on its surface which resists the passage of electrical current. When its surface is covered with an electrically conductive metal, however, this high resistivity is drastically reduced which can be used as an effective electrode. This feature of titanium has particular importance in the chromium plating art as contemplated by the invention because it has always been a problem to insulate and mask those portions of a typical lead anode which are not intended to act anodically in 3,247,083 Patented Apr. 19, 1966 the bath. Various dielectric materials are generally disposed on conventional lead anodes to achieve electrical insulation where desired, but they add further to the weight and size of the already massive all-lead anodes. Furthermore, the insulating material prevents the covered portions of the anode from being cooled by the electrolytic solution, and consequently the thinner anode sections are further weakened by over-heating. Hence, the titanium body structure contemplated by the invention offers a special advantage in chromium plating in that it requires no insulation to prevent it from acting anodically and is very effectively cooled because it is in direct contact with the electrolyte, thereby permittinga given anode section to carry considerably more current and be reduced in size and weight.

Lead coverings on titanium structures, as contemplated by this invention, produce unusual advantages in conjunction with chromic acid electrolytes which do not result from the use of other covering materials or other electrolytic solutions. For example, experiments have shown that when a titanium body structure is partly covered with platinum and is used in a typical chromic acid electrolyte, far more contamination occurs in the bath than is desirable for satisfactory operation. In particular, platinized titanium anodes in chromic acid electrolytes cause a critical rise in the amount of trivalent chromium released in the bath, probably because the platinum cannot effect sufficient reoxidation of the chromium cations into chromic acid at the cathode. This is undesirable because a concentration of trivalent chromium is particularly detrimental in increasing the resistivity of the bath. However, when lead is used on a titanium body structure as provided herein, such an increase in trivalent chromium cations does not occur to any great extent even over prolonged periods of time, and the conductivity of the electrolytic solution remains unaffected.

Compared to the all-lead anodes almost universally employed in the chromium plating art, this new anode structure provides many important improvements, most of which are of a mechanical nature. No longer are the effective anodic areas structurally supported by additional heavy sections of lead or reinforced by sections of more rigid materials which are subject to attack by the electrolyte and therefore coated with a sheath of dielectric material. Hence, high strength and rigidity per unit weigh and size are one of the primary advantages of the new anode over known lead anodes. Since the anode structure of the invention is lighter and stronger, it can be formed into intricate and more versatile shapes to electroplate many bores, cavities, recesses and the like in inaccessible regions of workpieces which heretofore could not be plated. Also, the effective anodizing areas of the new structures are assured to remain in the desired juxtaposition relative to the surfaces to be plated so that the deposits of chrome are uniform and dimensionally accurate. It is Well known that many all-lead anodes undergo creep and other deformation because they are not sufi'iciently rigid to be self-supporting, but the new lead-titanium anodes have more than enough strength to support their own weight and maintain accurate desired spacing relative to the workpiece.

A further advantage of the new anode over conventional all-lead anodes results from the fact that lead is subjected to slow but significant deterioration during operation which over prolonged periods of time causes additional contamination of the chromic acid electrolytic bath. Titanium, however, is not susceptible to any significant deterioration of this sort by the electrolyte. Since the standard all-lead anodes expose far more lead to the bath than do the titanium-lead anodes of the invention, a marked reduction in lead contamination has been noted during the operation of the new anodes.

The method of this invention is particularly advantageous for plating irregularly shaped metallic articles with chromium on various of their flat and curved surfaces which were in confined places too difiicult to reach with a heavy conventional lead anode. The lead covered titanium anode used in applicants plating method has the strength and rigidity required to produce an anode in intricate form. In making this anode, a titanium skeleton structure conformed to the Shape of the required anode is first fabricated with hooks provided thereon for its support. The lead covering is then provided on the desired surfaces, and may then be machined to the desired shape.

While any method that provides an adherent coating of lead on a titanium body structure can be used to produce the anode, we found it to be eminently suitable by casting a layer of lead on the titanium surface using a suitable solder. A particular solder that has been successfully used by us contains 4 parts by weight of commercial grade lead for making chromic plating anode, 1 part by weight of tin, 1 part by weight of zinc, and a small amount of sal ammoniac (ammonium chloride). Generally, the commercial grade lead contains about 6% of alloying additives consisting of antimony and zinc. In preparing the solder, the mixtures are first melted and heated to a straw color. After the dross on the top of the melt is removed, the solder is cast into rod shape. To use this particular solder, the titanium body structure is preferably chemically cleaned with acid or by liquid honing. After cleaning, the titanium body structure is heated to above the melting point of the solder and the solder stick is then applied onto the heated surface until an adherent layer of coating appears thereon. A layer of lead then is cast thereon. The lead used for the coating preferably contains about at least 4% of antimony or tin to provide a more resistant lead coating. The added alloying elements in-the lead lessen the chromate formation which substantially reduces the contamination of the plating bath.

For plating an irregularly shaped metallic article in accordance with the method of this invention, the article is first mounted in a rack designed to hold it securely in place beneath the surface of a chromic acid solution in a conventional electrolytic cell. The lead covered titanium anode prepared in accordance with the method described previously, is then lowered into the cell with the hook portion connected electrically to the positive terminal of the cell. Suitable fasteners are used to hold this anode rigidly in a juxtaposition with respect to the article in the cell so that it presents surfaces in opposition to all of the surfaces to be plated on the article. The article which is used as a cathode in the cell is then connected to the negative terminal of the cell to complete the electroplating circuits to provide a chromium deposit thereon. 1

The uncovered portion of the titanium body structures forms an insulating film readily in the chromium acid solutions and effectively prevents the passage of electric current into the bath which has the added advantage of allowing the great portion of the body structure to be cooled by direct contact with the electrolytic solutions. In plating according to this method, cation contamination was very little and, in particular, the trivalent chromium content was kept at a satisfactory minimum.

The over-all size and weight of an anode of this type made in accordance with the invention is considerably reduced from comparable anodes of known design. Since the greater part of the new anode is of titanium, a considerably more self-supporting metal than lead, there is no need to increase the mass of the new anode merely for purposes of adding to its strength. The titanium body structure provides a light skeleton for rigidly and accurately locating the relatively small amount of lead at the effective anodic areas and carries sutficient current to effect a highly satisfactory deposition of chromium on the workpiece without over-heating or contaminating the bath.

We claim:

A method for producing chromium electro-deposits on a base metal which comprises the steps of immersing as a cathode at least a portion of said base metal in a chromic acid electrolytic solution, positioning a nonconsumable anode in said solution in suitable juxta-position relative to said cathode, said anode having a titanium body structure and a layer of lead base alloy containing about 4% of an alloying additive selected from the group consisting of antimony and tin, said anode being prepared by initially coating a solder on a portion only of the surface area of said titanium body structure and subsequently casting a layer of lead onto only the soldercoated portions of said body structure to provide intimate electrical contact between said layer and said body structure both lead-covered and uncovered areas of the titanium body structure being immersed in said solution, whereby said lead covered area in the electrolytic solution serves as an effective anodic area, said solder comprising 4 parts by Weight of commercial lead, 1 part by weight of tin, 1 part by weight of zinc, and a small amount of sal ammoniac completing the electrodepositing circuit and depositing chromium onto said base metal.

References Cited by the Examiner UNITED STATES PATENTS 2,392,871 l/1946 Wick 204-l05 2,873,233 2/ 1959 Schnable 204-290 2,929,769 3/ 1960 Newell et a1 204290 2,987,453 1/1961 DuRose 204-290 FOREIGN PATENTS 221,757 5/1959 Australia.

JOHN H. MACK, Primary Examiner. 

